A state of emergency was declared after the 5km-deep tremor levelled high-rise buildings, tore up pavements and sprayed rubble on to the streets below.

A British aid team was due to leave Gatwick to fly to New Zealand’s South Island this morning and join the rescue mission, which has been put at risk by a series of smaller aftershocks.

The British Foreign Office is "urgently" seeking information about any UK casualties, but it is not yet known if any Britons are among the dead.

A spokesman said: "We are in close touch with the local authorities and are urgently seeking information. The High Commission in Wellington have mobilised a consular response and stand ready to provide any consular assistance that is required."

Prime Minister David Cameron yesterday telephoned his New Zealand counterpart John Key to pledge his support.

A national state of emergency was declared.

A Downing Street spokesman said: "They discussed the latest situation on the ground, and the Prime Minister reiterated Britain’s sympathies and condolences at this difficult time."

Rescuers are concentrating their efforts on the ruins of around a dozen buildings that collapsed or were badly damaged in the city.

It is the second time in six months Christchurch has been hit by a huge quake.

In September last year it was hit by a large tremor 19 miles out of the city.

Christchurch mayor Bob Parker said 120 people were rescued as teams worked through the night, while more bodies were also recovered.